DDD & REST — Domain-Driven APIs for the web

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Unless otherwise indicated, these slides are © 2013-2016 Pivotal Software, Inc. and licensed under a Creative Commons Attribution-NonCommercial license: http://creativecommons.org/licenses/by-nc/3.0/ DDD & REST  Domain-Driven APIs for the web Oliver Gierke / olivergierke ogierke@pivotal.io

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4 Spring Data Repositories &  Aggregates Spring HATEOAS Hypermedia  for Spring MVC Spring Data REST

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When designing REST APIs, what can we learn from DDD? “ 5

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What does it take to bridge the worlds of DDD & REST? “ 6

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8 http://www.infoq.com/minibooks/domain-driven-design-quickly

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Implementing Domain-Driven Design 10

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Value objects 11

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Stringly typed code 12 public class Customer { private Long id; private String firstname, lastname, email; … }

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Stringly typed code 13 public class SomeService { public void createUser(String firstname,  String lastname, String email) { … } }

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14 public class Customer { private Long id; private Firstname firstname; private Lastname lastname; private EmailAddress emailAddress; … }

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Value Objects are a  PITA to build in  some languages. 15

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Still, they’re worth it. 16 See „Power Use of Value Objects in DDD“ by Dan Bergh Johnsson.

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Lombok — putting the spice back into Java. 17

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18 @Value public class Customer { UUID id = UUID.randomUUID(); Firstname firstname; Lastname lastname; EmailAddress email; @Value static class EmailAddress { String value; } }

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AutoValue 19 Project website on GitHub.

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Entities & Repositories 20

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21 Order LineItem Product Invoice Customer Payment Address Email

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21 Order LineItem Product Invoice Customer Payment Address Email

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Entity +  Repository =   Aggregate 22

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Aggregates.  Scopes of consistency. 23

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24 Order LineItem Product Invoice Customer Payment Address Email

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Aggregates.  The key things  to refer to. 25

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26 Order LineItem Product Invoice Customer Payment Address Email

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Don’t get trapped by datastore thinking. 27

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28 Order LineItem Product Invoice Customer Payment Address Email

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Bounded Context 29

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Order LineItem Product Invoice Customer Payment Address

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31 Shipping Accounting Catalog Orders User  Registration

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31 Shipping Accounting Catalog Orders User  Registration Accounting Payment information Billing Address Shipping Shipping address Customer Product

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Domain Events 32

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33 Level 0: No events at all

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33 Level 0: No events at all Level 1: Explicit operations

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If you’re calling two setters in a row, you’re missing a concept. 34

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35 Level 0: No events at all Level 1: Explicit operations Level 2: Some operations as events

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Domain events as  state transitions. 36

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Expose important  events to interested parties via feeds. 37

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38 Level 0: No events at all Level 1: Explicit operations Level 2: Some operations as events Level 3: Event Sourcing

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REST 39

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REST ≠   CRUD via HTTP 40

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Representation design matters 41

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Value objects  need custom   serializers. 42

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Aggregates Identiﬁable  Referable  Scope of consistency 43

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Resources Identiﬁable  Referable  Scope of consistency 44

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Hypermedia 45

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Serving data and navigation information  at the same time. 46

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Trading domain knowledge with protocol complexity in clients. 47

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48 Amount of domain knowledge in the client Amount of protocol knowledge in the client Coupling to the server Non-hypermedia  based systems Hypermedia  based systems

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API evolvability is key  in a system of systems 49

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50 GET /orders { „_links“ : { „cancel“ : { „href“ : … }, … „createdDate“ : …, „status“ : „Payment expected“ … }

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51 GET /orders { „_links“ : { „cancel“ : { „href“ : … }, … „createdDate“ : …, „status“ : „Payment expected“ … }

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52 “Internationalize  all user facing text.

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Aaaargh! 53

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54 GET /orders { „_links“ : { „cancel“ : { „href“ : … }, … „createdDate“ : …, „status“ : „Payment expected“ … }

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Reducing decisions in clients to whether a  link is present or not. 55

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Prefer explicit   state transitions over poking at your resources using PATCH. 56

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Translate domain concepts into web- appropriate ones. 57

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58 Aggregate Root / Repository Collection / Item Resources IDs URIs @Version ETags Last Modified Property Last Modified Header Relations Links

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RESTBucks payment expected preparing cancelled ready completed 1 2 3 4 5 6

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Method URI Action Step POST /orders Create new order 1 POST/PATCH /orders/{id} Update the order (only if "payment expected") 2 DELETE /orders/{id} Cancel order (only if "payment expected") 3 PUT /orders/{id}/payment Pay order (only if "payment expected") 4 Barista preparing the order GET /orders/{id} Poll order state 5 GET /orders/{id}/receipt Access receipt DELETE /orders/{id}/receipt Conclude the order process 6

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Method Resource type Action Step POST orders Create new order 1 POST/PATCH update Update the order 2 DELETE cancel Cancel order 3 PUT payment Pay order 4 Barista preparing the order GET order Poll order state 5 GET receipt Access receipt DELETE receipt Conclude the order process 6

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Spring RESTBucks 62

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Web Service Repository - Orders Spring Data Spring Data  REST Payment Spring Data Manual  implementation Manual  implementation

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JacksonCustomizations Externalize tweaks to the general JSON design 64

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Spring Data REST  for the CRUDdy parts. 65

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ResourceProcessor To conditionally sneak links into the default representation. 66

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Resources 67 Spring RESTBucks  https://github.com/olivergierke/spring-restbucks Benefits of Hypermedia APIs  http://olivergierke.de/2016/04/benefits-of-hypermedia/index.html

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Questions? 68